1. Field of the Invention
The present invention relates to a wavelength division multiplexer.
2. Description of Related Art
A dielectric multilayer film is in wide use as an antireflection film coated on an eyeglass lens, a color separation filter for TV coated on a glass substrate, and so on. For example, the dielectric multilayer film is used for a color separation filter used in a liquid crystal projector, camera, and so on. It is also used for a laser detection mirror in a digital versatile disk (DVD) device and so on. When use for the laser detection mirror, the dielectric multilayer film is placed between two glass prism substrates so that light is incident on the dielectric multilayer film at a certain angle.
In the field of communications, wavelength division multiplexing (WDM) optical communication technology has been introduced. This technique needs a filter where a dielectric multilayer film which serves as an edge filter or a bandpass filter is deposited on a glass substrate in order to separate light with different wavelengths.
In the optical communication, it is possible to combine or separate light with a plurality of wavelength bands by connecting 3-terminal modules in cascade. It is, however, necessary to use a plurality of 3-terminal modules of at least one less than the number of wavelength bands, which requires high equipment costs, a large area, and high installation costs.
Japanese Unexamined Patent Application Publication No. 08-082711 and Yoichi Fujii, “Opto-electronics”, Agne Shofu Publishing, Inc., Tokyo, 1993, p. 169 introduce a technique that incorporates a plurality of bandpass filters and edge filters into one module to combine and separate a plurality of wavelengths. However, since a split angle of light is small, incorporation of a transmitting laser and a receiving diode requires a long optical path, which results in a large equipment size and high installation costs.
On the other hand, to reduce the equipment size, it is necessary to use a laser diode array, resulting in cost increase. In order to reduce the equipment size without increasing costs, it is necessary to increase the split angle of light. This, however, causes large divergence between P-wave and S-wave of outgoing light, deteriorating division multiplexer characteristics. The characteristics thus deteriorate if the P-polarized and S-polarized light enter at a large split angle, which is, at a large incident angle. The amplitude/wavelength characteristics of outgoing light thereby greatly diverge due to the polarization direction of the incoming light.
A technique to overcome this problem is proposed in Japanese Unexamined Patent Application Publication No. 2000-162413. This technique uses Si for a high refractive index layer of a dielectric multilayer filter, and TiO2, SiO2 for a low refractive index layer of the filter. However, when the filter is placed under high temperature and humidity conditions of 85° C.85% RH, oxygen of TiO2 and SiO2 of the low refractive index layer spreads to the high refractive index layer side. This causes the refractive index of Si and Ge layer to decrease and that of the low refractive index layer to increase. This results in wavelength shift and optical characteristics variation. Further, since ZnS and ZnSe, which are generally used for the high refractive index layer, show low adhesion to SiO2 and TiO2, ZnS and ZnSe are easy to separate from SiO2 and TiO2.
If incident medium is air with refractive index 1, the characteristics variation due to a difference in polarization direction can be reduced. However, recent optical components are packaged in high density for miniaturization, and the filter is usually directly bonded to other optical parts such as fiber capillary, prism, lens, and waveguide. In this case, in order to have air as the incident medium, it is necessary to form an air-sandwich structure. To form the air-sandwich structure, an antireflection layer is formed on a bonded surface so as to suppress the amplitude variation due to multiple reflection. Since the antireflection layer is optimized for air with refractive index 1, the transmission characteristics decreases if a resin and the like used for bonding comes to a light transmitting surface. It is therefore necessary to make a structure that prevents the resin from coming to the surface, resulting in cost increase.